1. Field of the Invention
The present invention is related to a valve timing adjuster that changes valve timing of opening and closing at least one of an intake valve and an exhaust valve.
2. Description of Related Art
A conventional vane-type valve timing adjuster opens and closes at least one of an intake valve and an exhaust valve based on a phase difference. The valve timing adjuster generates the phase difference based on a relative rotation between (a) a camshaft and (b) a timing pulley, or a relative rotation between (a) the camshaft and (b) a sprocket, by driving the camshaft through the timing pulley or the sprocket. Typically, the timing pulley and the sprocket rotate synchronously with the crankshaft of an internal combustion engine.
A “pulley-type valve timing adjuster” has a timing belt and a timing pulley. The timing belt serves as a transmission device that transmits a driving force, and the timing pulley serves as a receiving structure that receives the driving force. In contrast, a “sprocket-type valve timing adjuster” has a chain and a sprocket. The chain serves as a transmission device that transmits a driving force, and the sprocket serves a receiving structure that receives the driving force. The timing belt employed in the pulley-type valve timing adjuster is usually made of a rubber, and has projections and recesses at an inner side thereof. Also, the timing pulley has “pulley teeth” at an outer periphery thereof, and the pulley teeth engage with the projections and the recesses of the timing belt. In contrast, the chain employed in the sprocket-type valve timing adjuster is usually made of iron, and the sprocket has gear teeth formed at an outer periphery thereof. The chain engages with the gear teeth of the sprocket.
Because the timing belt is made of the rubber, the timing belt is silently operable and is light weight compared with the iron chain. Also, because the timing belt is light weight, it is possible to further improve the fuel efficiency when mounted on a vehicle.
In general, because the timing belt is wider than the chain, a pulley part, which has a required certain width, is to be provided at an outer peripheral part of the valve timing adjuster. In the above design, the pulley inevitably has a cup shape, and the cup-shaped pulley is provided to cover the outer periphery of the housing of the valve timing adjuster from the rear side of the housing. An example of the above cup shape is described in JP-A-2008-204735.
If the pulley has the cup shape, the timing belt, which engages with the outer periphery of the pulley, radially inwardly applies load to the outer wall of the pulley, resulting in the deformation of the pulley. When the pulley is leaning due to the deformation caused by the above load, the timing belt may be erroneously displaced. Thus, in order to prevent the deformation, it is required to make the outer wall thicker, and thereby the valve timing adjuster becomes greater in weight disadvantageously. Therefore, even when the timing belt is light weight, the total weight of the valve timing adjuster having the timing belt and the pulley becomes greater. As a result, the cup-shaped pulley may degrade the fuel efficiency when mounted on the vehicle.
Furthermore, the timing belt has poor durability although the timing belt has the certain advantages as above. For example, the timing belt is easy to deteriorate when subjected to oil. Thus, it is required to prevent the leakage of a small amount of oil out of the pulley-type valve timing adjuster even though the above amount of oil may be acceptable in the sprocket-type valve timing adjuster.
For example, if a porous metal sintered body is used for the outer casing component of the valve timing adjuster, oil may leak. In order to prevent the leakage, a certain process, such as a sealing process or a resin impregnation process, is required for the sintered body component. As a result, the manufacturing cost increases.
Furthermore, instead of using a porous component for the outer casing component of the valve timing adjuster, a component made by machining a steel product may be alternatively employed for the prevention of the oil leakage. However, in the above alternative case, the product weight may be increased, or the manufacturing cost may increase.